The present invention relates to a metal halide lamp, i. e., a high-intensity discharge lamp which emits light by discharge in a mixture of metal vapors and dissociated products of halides.
A molybdenum foil is generally provided as a conductor at a sealing portion of a single-tube high-watt metal halide lamp (hereinafter simply referred to as "lamp"), a kind of high-intensity discharge lamp. The outer covering of the sealing portion in this kind of lamp has been formed of a transparent material such as quartz. The distal end of the molybdenum foil in the sealing portion away from the center of the discharge tube is exposed to the air.
In the conventional lamp having the above-mentioned structure, when the lamp is burnt, the temperature of the molybdenum foil is raised abruptly by radiation heat from the lamp, conduction heat transferred through the sealing portion, heat generation due to the resistance of the molybdenum foil itself, etc.
Since the distal end of the molybdenum foil in the sealing portion away from the center of the discharge tube is heated at a high temperature in the air as described above, the molybdenum foil is apt to be oxidized. When this kind of lamp is burnt for a considerable period of time, the molybdenum foil is oxidized, and the sealing portion is deteriorated, whereby the service life of the lamp is shortened.
In order to prevent the above-mentioned molybdenum foil from being oxidized, it was necessary to lower the temperature of the sealing portion to 350.degree. C. or less, while the lamp was burnt.
However, in such a conventional lamp consuming high power, its sealing portion is heated at a high temperature. Therefore, it was difficult to lower the temperature of the molybdenum foil to 350.degree. C. or less.
In order to solve the above-mentioned problems, various cooling means have been taken for the conventional lamp. For example, its caps are provided with heat radiation fins, or its sealing portions are extremely lengthened so as to locate the molybdenum foils at the ends of the sealing portions away from the light-emitting portion.
It is necessary to have the above-mentioned cooling means for a lamp consuming high power. In particular, long sealing portions have been used for a lamp consuming 1800 W or more.
However, since the cap of the conventional lamp provided with heat radiation fins is complicated in shape, the production cost of the cap increases. In addition, the lamp provided with extremely lengthened sealing portions is difficult to produce and becomes large.
These problems have raised the production cost of the lamp. In particular, the pinch sealing method being advantageous for reducing the production cost cannot be used for such a lamp having extremely lengthened sealing portions.